There are two types of flatspins and they are called the same thing and it is irritating. One is based off a flat sideflip (which is what sammy carlson is a monster at). The other is based off a backflip thrown off axis. I'm calling the inverted one a flackspin and the flat one a flatspin. /trick name argument
But if you're doing a backflip, go for the flackspin, i should be really easy.